1,131 research outputs found
An X-ray Survey in SA 57 with XMM-Newton
The maximum number density of Active Galactic Nuclei (AGNs), as deduced from
X-ray studies, occurs at z<~1, with lower luminosity objects peaking at smaller
redshifts. Optical studies lead to a different evolutionary behaviour, with a
number density peaking at z~2 independently of the intrinsic luminosity, but
this result is limited to active nuclei brighter than the host galaxy. A
selection based on optical variability can detect low luminosity AGNs (LLAGNs),
where the host galaxy light prevents the identification by non-stellar colours.
We want to collect X-ray data in a field where it exists an optically-selected
sample of "variable galaxies'', i.e. variable objects with diffuse appearance,
to investigate the X-ray and optical properties of the population of AGNs,
particularly of low luminosity ones, where the host galaxy is visible. We
observed a field of 0.2 deg^2 in the Selected Area 57, for 67ks with
XMM-Newton. We detected X-ray sources, and we correlated the list with a
photographic survey of SA 57, complete to B_J~23 and with available
spectroscopic data. We obtained a catalogue of 140 X-ray sources to limiting
fluxes 5x10^-16, 2x10^-15 erg/cm^2/s in the 0.5-2 keV and 2-10 keV
respectively, 98 of which are identified in the optical bands. The X-ray
detection of part of the variability-selected candidates confirms their AGN
nature. Diffuse variable objects populate the low luminosity side of the
sample. Only 25/44 optically-selected QSOs are detected in X-rays. 15% of all
QSOs in the field have X/O<0.1.Comment: 13 pages, 6 figures, 4 tables, A&A in pres
Blazar surveys with WMAP and Swift
We present the preliminary results from two new surveys of blazars that have
direct implications on the GLAST detection of extragalactic sources from two
different perspectives: microwave selection and a combined deep X-ray/radio
selection. The first one is a 41 GHz flux-limited sample extracted from the
WMAP 3-yr catalog of microwave point sources. This is a statistically well
defined sample of about 200 blazars and radio galaxies, most of which are
expected to be detected by GLAST. The second one is a new deep survey of
Blazars selected among the radio sources that are spatially coincident with
serendipitous sources detected in deep X-ray images (0.3-10 keV) centered on
the Gamma Ray Bursts (GRB) discovered by the Swift satellite. This sample is
particularly interesting from a statistical viewpoint since a) it is unbiased
as GRBs explode at random positions in the sky, b) it is very deep in the X-ray
band (\fx \simgt \ergs) with a position accuracy of a few
arc-seconds, c) it will cover a fairly large (20-30 square deg.) area of sky,
d) it includes all blazars with radio flux (1.4 GHz) larger than 10 mJy, making
it approximately two orders of magnitude deeper than the WMAP sample and about
one order of magnitude deeper than the deepest existing complete samples of
radio selected blazars, and e) it can be used to estimate the amount of
unresolved GLAST high latitude gamma-ray background and its anisotropy
spectrum.Comment: 3 pages, 3 figures, to appear in Proc. of the 1st GLAST Symposium,
Feb 5-8, 2007, Stanford, AIP, Eds. S. Ritz, P. F. Michelson, and C. Meega
Evidence for T Tauri-like emission in the EXor V1118 Ori from near-IR and X-ray data
We present a near-IR study of the EXor variable V1118 Ori, performed by
following a slightly declining phase after a recent outburst. In particular,
the near-IR (0.8 - 2.3 micron) spectrum, obtained for the first time, shows a
large variety of emission features of the HI and HeI recombination and CO
overtone. By comparing the observed spectrum with a wind model, a mass loss
rate value is derived along with other parameters whose values are typical of
an accreting T Tauri star. In addition, we have used X-ray data from the XMM
archive, taken in two different epochs during the declining phase monitored in
IR. X-ray emission (in the range 0.5 - 10 keV) permits to derive several
parameters which confirm the T Tauri nature of the source. In the near-IR the
object maintains a low visual extinction during all the activity phases,
confirming that variable extinction does not contribute to brightness
variations. The lack of both a significant amount of circumstellar material and
any evidence of IR cooling from collimated jet/outflow driven by the source,
indicates that, at least this member of the EXor class, is in a late stage of
the Pre-Main Sequence evolution. In the X-ray regime, an evident fading is
present, detected in the post-outburst phase, that cannot be reconciled with
the presence of any absorbing material. This circumstance, combined with the
persistence (in the pre- and post-outburst phases) of a temperature component
at about 10 MK, suggests that accretion has some influence in regulating the
coronal activity
Copulas, credit portfolios, and the broken heart syndrome
David X. Li is professor of Finance at the Shanghai Advanced Institute of Finance (SAIF). For more than two decades, he worked at leading nancial institutions in the areas of product de- velopment, risk management, asset/liability management, and investment analytics. He was the chief risk o cer for China International Capital Corporation (CICC) Ltd, head of credit derivative research and analytics at Citigroup and Barclays Capital, and head of modeling for AIG Invest- ments.
David has a PhD degree in Statistics from the University of Waterloo, Masters degrees in Eco- nomics, Finance, and Actuarial Science, and a Bachelor\u2019s degree in Mathematics. David is currently an Associate Editor for the North American Actuarial Journal, an adjunct professor at the University of Waterloo, a senior research fellow at Global Risk Institute in Toronto, and a senior advisor to the Risk Management Institute at the National University of Singapore. David was one of the pioneers in credit derivatives. His seminal work of using copula functions for credit port- folio modeling has been widely cited by academic research, broadly used by practitioners for credit portfolio trading, risk management and rating, and well covered by the media (Wall Street Journal, Financial Times, Nikkei, and CBC News)
Detailed Shape and Evolutionary Behavior of the X-ray Luminosity Function of Active Galactic Nuclei
We construct the rest-frame 2--10 keV intrinsic X-ray luminosity function of
Active Galactic Nuclei (AGNs) from a combination of X-ray surveys from the
all-sky Swift BAT survey to the Chandra Deep Field-South. We use ~3200 AGNs in
our analysis, which covers six orders of magnitude in flux. The inclusion of
the XMM and Chandra COSMOS data has allowed us to investigate the detailed
behavior of the XLF and evolution. In deriving our XLF, we take into account
realistic AGN spectrum templates, absorption corrections, and probability
density distributions in photometric redshift. We present an analytical
expression for the overall behavior of the XLF in terms of the
luminosity-dependent density evolution, smoothed two power-law expressions in
11 redshift shells, three-segment power-law expression of the number density
evolution in four luminosity classes, and binned XLF. We observe a sudden
flattening of the low luminosity end slope of the XLF slope at z>~0.6. Detailed
structures of the AGN downsizing have been also revealed, where the number
density curves have two clear breaks at all luminosity classes above log LX>43.
The two break structure is suggestive of two-phase AGN evolution, consisting of
major merger triggering and secular processes.Comment: 39 Pages, 9 figures. ApJ in pres
The HELLAS2XMM survey. IX. Spectroscopic identification of super-EROs hosting AGNs
We present VLT near-IR spectroscopic observations of three X-ray sources
characterized by extremely high X-ray-to-optical ratios (X/O>40), extremely red
colors (6.3<R-K<7.4, i.e. EROs) and bright infrared magnitudes (17.6<K<18.3).
These objects are very faint in the optical, making their spectroscopic
identification extremely challenging. Instead, our near-IR spectroscopic
observations have been successful in identifying the redshift of two of them
(z=2.08 and z=1.35), and tentatively even of the third one (z=2.13). When
combined with the X-ray properties, our results clearly indicate that all these
objects host obscured QSOs (4e44 < L(2-10keV) < 1.5e45 erg/s, 2e22 < N_H < 4e23
cm-2) at high redshift. The only object with unresolved morphology in the K
band shows broad Halpha emission, but not broad Hbeta, implying a type 1.9 AGN
classification. The other two objects are resolved and dominated by the host
galaxy light in the K band, and appear relatively quiescent: one of them has a
LINER-like emission line spectrum and the other presents only a single, weak
emission line which we tentatively identify with Halpha. The galaxy
luminosities for the latter two objects are an order of magnitude brighter than
typical local L* galaxies and the derived stellar masses are well in excess of
10^11 Msun. For these objects we estimate black hole masses higher than 10^9
Msun and we infer that they are radiating at Eddington ratios L/L_Edd < 0.1. We
discuss the implications of these findings for the coevolution of galaxies and
black hole growth. Our results provide further support that X-ray sources with
high X/O ratios and very red colors tend to host obscured QSO in very massive
galaxies at high redshift.Comment: 7 pages, 5 figures, accepted for publication in A&
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